Fix CSV export of multi-repo Page Provider results

Steps to reproduce:

1. Set up a Nuxeo instance configured with multiple (2+) repositories (see example configuration attached using two Mongo repositories & ES indices)
2. Install nuxeo-csv addon to enable CSV import and export
3. Configure an ES-enabled Page Provider to return results from both repositories by checking the "Multi repository" flag under Advanced Configuration
4. In Web UI, open the search view for the multi-repo PP and run
5. From the actions bar, click the nuxeo-csv-export-button to download a .csv export of the results

Expected behavior: the output .csv shows all results from running the multi-repo PP

Actual behavior: the output .csv only shows results records corresponding to the current browsing repository